Colorado Rockies catcher Eliezer Alfonzo was enhancing drugs.
Alfonzo, who also tested positive in 2008, would get a lifetime ban for a third drug violation. He said through the Major League Baseball Players Association that he will fight the suspension.
"I am surprised by this positive test," Alfonzo said in a statement. "I learned my lesson in 2008 and have not taken any prohibited substances since then. With the union's help, I intend to fight this suspension and look forward to appearing before the arbitrator in the near future."
The MLB Players Association said it would "do its best to preserve and protect the rights granted to Eliezer under the terms of the Joint Drug Agreement."
Alfonzo was batting .267 in 25 games for the Rockies this season and last appeared as a pinch hitter Tuesday in Milwaukee.
He was the second player to be faced with a 100-game suspension this season. Manny Ramirez retired from the Rays in April rather than accept his 100-game suspension for a second steroids violation.